Testosterone Biology

• Treatment of hypogonadism caused by an underlying medical condition (FDA-approved indication)
• Restoration of physiological testosterone in confirmed deficiency
• Symptom evaluation for hypogonadism (libido, energy, mood, with the caveat that secondary symptoms are nonspecific)

Hypogonadism from an underlying medical condition

The only FDA-approved indication: testicular failure (primary) or pituitary/hypothalamic disease (secondary), confirmed by low morning testosterone with symptoms

NOT age-related decline alone

Since 2015, FDA labeling explicitly states testosterone products are not approved for low testosterone due solely to aging

Testosterone is biosynthesized from cholesterol through a defined steroidogenic pathway: Cholesterol to Pregnenolone (the rate-limiting step, via the side-chain cleavage enzyme) to DHEA to Androstenedione to Testosterone. From testosterone, two key downstream conversions occur: reduction to dihydrotestosterone (DHT) by 5-alpha-reductase, and aromatization to estradiol by the aromatase enzyme. Testosterone acts on the androgen receptor in target tissues; DHT is a more potent androgen-receptor agonist in skin, prostate, and hair follicles; estradiol mediates bone density and aspects of libido and feedback in men. In circulation, ~40-45% is bound to sex hormone-binding globulin (SHBG, biologically inactive), ~50-55% is loosely bound to albumin (weakly available), and only ~2-3% circulates free and immediately bioactive. Total testosterone is the sum of all fractions; bioavailable testosterone is free plus albumin-bound.

Testosterone (17-beta-hydroxyandrost-4-en-3-one) is a C19 steroid built on the four-ring gonane (cyclopentanoperhydrophenanthrene) skeleton shared by all steroid hormones. It carries a ketone at C3, a double bond between C4 and C5, a hydroxyl at C17, and angular methyl groups at C18 and C19 (the latter is the defining 19th carbon of the androstane series). Esterification of the 17-beta-hydroxyl with fatty acids, cypionate, enanthate, propionate, undecanoate, produces the depot prodrugs used clinically, slowing release from the injection or oral depot and extending the effective half-life.

Free fraction

~2-3% (bioactive)

SHBG-bound

~40-45% (inactive)

Albumin-bound

~50-55% (weakly available)

Diurnal peak

Morning (7-10 AM)

Diurnal trough

Afternoon/evening

• Two separate morning total testosterone measurements before diagnosis
• Free testosterone (preferably by LC-MS/MS or calculated with SHBG)
• LH and FSH (to distinguish primary vs secondary hypogonadism)
• SHBG
• Prolactin
• Hematocrit / CBC at baseline (polycythemia is the main physiological risk on therapy)
• PSA in men over 40
• Blood pressure (especially with oral formulations)

Brown-Sequard's 1889 self-injection of animal testicular extract foreshadowed the field. Testosterone was isolated and chemically synthesized in 1935 (Butenandt and Ruzicka, who shared a Nobel Prize in 1939 for steroid work). Clinical androgen replacement for hypogonadism followed mid-century. The modern marketplace story turns on advertising: Abbott/AbbVie's AndroGel and the 'Low T' campaign, launched in 2008, reframed age-related symptoms as a treatable deficiency and drove a prescribing surge. In 2015 the FDA ordered new labeling restricting approval to hypogonadism from an underlying medical condition and rebuked the 'Low T' framing as misleading. The cardiovascular debate, fueled by observational studies suggesting harm, culminated in the TRAVERSE trial (2023) and the February 2025 FDA removal of the cardiovascular black-box warning.

1. 1889Brown-Sequard self-injects animal testicular extract
2. 1935Testosterone isolated and synthesized (Butenandt, Ruzicka)
3. 1939Nobel Prize in Chemistry for steroid hormone work
4. 2002NERI study reports ~1%/year decline, later the basis of 'Low T' marketing
5. 2008AbbVie 'Low T' AndroGel campaign launches
6. 2015FDA orders labeling restriction (hypogonadism from medical condition only) and rebukes 'Low T' advertising
7. 2018Endocrine Society guideline (Bhasin et al.) sets diagnostic criteria
8. 2023TRAVERSE trial: no increase in major adverse cardiovascular events
9. 2025FDA removes cardiovascular black-box warning, adds blood-pressure monitoring (February)

• Does treating age-related (functional) hypogonadism, as opposed to classical/organic hypogonadism, yield net long-term benefit?
• What total and free testosterone thresholds best define clinically meaningful deficiency across age groups?
• Should free or bioavailable testosterone replace total testosterone as the primary diagnostic metric?
• How much does the unexplained secular decline in male testosterone reflect biology versus measurement and cohort differences?
• Can a standardized harmonized reference range reconcile discrepant lab cutoffs across assays and populations?

Selective androgen receptor modulators (SARMs)Investigational tissue-selective agents aiming for anabolic effect with fewer androgenic and prostate effects; none FDA-approved for hypogonadism.Oral testosterone formulationsNewer oral undecanoate products seek to avoid injection while managing absorption and blood-pressure signals.Assay harmonizationCDC and standardization programs working to align LC-MS/MS reference ranges across laboratories.Functional hypogonadism reversalGrowing interest in treating obesity, sleep apnea, and metabolic drivers to restore endogenous testosterone before lifelong therapy.

Synthesis site

Leydig cells (~95%), adrenal cortex (~5%)

Active fraction

Free testosterone ~2-3%

Normal total T

300-1000 ng/dL (10.4-34.7 nmol/L)

Gold-standard assay

LC-MS/MS

Peak timing

Early morning, 7-10 AM

Age decline

~1-2%/year after age 30

What follows is unfinished work on our side, not a finding about Testosterone Biology. A short entry is not a safe entry.

• Cost and market. We have not documented what it sells for or who sells it.
• Supply chain. We have not mapped where it is made.
• Who uses it. We have not documented the user groups.
• Children and teenagers. We have not checked what is known about younger people.
• What can go wrong. We have not written the risk line for this one. Read the safety section below rather than treating its absence as reassurance.